First Grammatical Treatise

The First Grammatical Treatise (Icelandic: Fyrsta Málfræðiritgerðin) is a 12th-century work on the phonology of the Old Norse or Old Icelandic language. It was given this name because it is the first of four grammatical works bound in the Icelandic manuscript Codex Wormianus. The anonymous author is today often referred to as the "First Grammarian".

The First Grammatical Treatise was of great interest to some mid-20th century linguists, since it systematically used the technique of minimal pairs to establish the inventory of distinctive sounds or phonemes in the Icelandic language, in a manner reminiscent of the methods of structural linguistics. It is also notable for revealing the existence of a whole series of nasal vowel phonemes, whose presence in the Icelandic language of the time would otherwise be unknown.